Semiconductor films and insulating films as the constituents of semiconductor devices and electronic circuits (particularly, VLSIs) are formed by various film-forming methods. Such film-forming method includes vacuum evaporation process, sputtering process, CVD process, and plasma CVD process. Of these film-forming processes, the plasma CVD process has the most popularly used since it is advantageous in that the quality of a film to be formed can be relatively easily controlled as desired. For instance, the plasma CVD process has been generally employed in the case of forming a thin film such as Si films used as the semiconductor layer, SiN films used as the protective film, and SiO.sub.2 films used as the interlayer dielectric film of a semiconductor device.
In the case of forming a deposited film by way of the plasma CVD process, the film formation is generally conducted while contacting a substrate on which the deposited film is to be formed with plasma having a plasma density of 1.times.10.sup.10 /cm.sup.3 or more, wherein a RF (Radio Frequency) with a frequency of 13.56 MHz or a microwave with a frequency of 2.45 GHz is used as the excitation source for producing said plasma. Now, the plasma CVD process with the use of a RF has advantages in that plasma can be relatively easily stabilized and a deposited film of relatively good quality can be obtained but it is accompanied by a disadvantage in that a high deposition rate is hardly attained. On the other hand, as for the plasma CVD process with the use of a microwave, it has advantages in that raw material gas is decomposed at a high efficiency and a high deposition rate can be attained but it is accompanied by a disadvantage in that it is difficult to stably form a deposited film of good quality while maintaining plasma in a stable state.
In these plasma CVD processes, ions in the plasma are accelerated by a sheath electric field formed at a contact face between the substrate and the plasma and they are radiated to the substrate at an energy of some tens to 100 eV, and because of this, a deposited film formed on the substrate unavoidably suffers from a so-called plasma damage to a certain extent. Further, in these plasma CVD processes using organic gas such as tetraethoxysilane, tetramethoxysilane, or the like as the film-forming raw material gas, there are problems such that undesirable reactions such as dissociation of C--H bond are occurred, resulting in causing contamination of carbon atoms dissociated into the deposited film formed, and because of this, it is difficult to stably form the deposited film in high quality.
In order to solve these problems, there has been proposed a so-called remote plasma CVD process wherein a plasma generation chamber is isolated from a film-forming chamber in which a substrate on which a film is to be formed to prevent a deposited film formed on the substrate from suffering from such a plasma damage as above described. An example of such remote plasma CVD process can be found in U.S. Pat. No. 4,066,037. The plasma CVD apparatus disclosed in this patent literature is of the constitution shown in FIG. 4. The plasma CVD apparatus shown in FIG. 4 comprises a deposition chamber 401 having a plasma generating coil 408 wound around the exterior thereof, wherein the coil 408 is electrically connected to a RF power source (not shown). Reference numeral 402 indicates an electric heater disposed in the inside of the film-forming chamber 401. Reference numeral 405 indicates a substrate on which a film is to be formed which is arranged on the heater 402. The deposition chamber 401 is provided with an exhaust pipe connected to a vacuum pump which serves to evacuate the inside of the deposition chamber 401. Each of reference numerals 403 and 404 indicates a gas feed pipe. Gas supplied through the gas feed pipe 403 is spouted out through gas liberation holes 406 provided at a gas dispersion head 409 disposed in the deposition chamber 401. On the other hand, gas supplied through the gas feed pipe 404 is spouted out into the deposition chamber 401 through a gas dispersion device 407 which is disposed at an upper position of the deposition chamber 401.
The gas supplied through the gas feed pipe 403 includes monosilane gas (SiH.sub.4). The gas supplied through the gas feed pipe 404 includes nitrogen gas (N.sub.2) and argon gas (Ar).